This adds a task which builds the site with the URL of
'https://f-droid.org', and also copies the output to a directory
specified by the environment variable $DEPLOY_DIR. The goal is to run
the following script:

 gitlab-runner exec docker f-droid.org \
   -docker-volumes `pwd`/_site:/builds/output \
   --env DEPLOY_DIR=/builds/output

to make the available to the Docker host machine after running.